Plasma volume, osmolality, vasopressin, and renin activity during graded exercise in man

Abstract

Plasma volume (PV), renin activity (PRA) and osmotic (Osm), Na+ and arginine vasopressin (AVP) concentrations were measured in venous blood samples taken before and after 3 levels of cycle ergometer exercise (100, 175 and 225 W) in 15 young male volunteers. Plasma volume and solute concentrations changed significantly (P < 0.05, denoted by *) with work intensity. The %.DELTA.PV was -3.7%* at 100 W, -8.8%* at 175 W and -12.4%* at 225 W. Plasma Na+ concentration, Osm and AVP increase were curvilinear with graded exercise and were significant only when work intensity exceeded 40% .ovrhdot.VO2 max [maximum O2 consumption]. PRA increase was linear and significant at all work levels. The %.DELTA.PV was significantly correlated with .DELTA.Osm (r = 0.99*) and .DELTA.Na+ (r = 0.89*) but had low concentrations with .DELTA.AVP (r = 0.22, NS) and .DELTA.PRA (r = 0.12, NS). .DELTA.AVP was significantly correlated with .DELTA.Na+ (r = 0.86*) and .DELTA.Osm (r = 0.83, NS); .DELTA.PRA had low correlations with .DELTA.Na+ (r = 0.33, NS), .delta.Osm (r = 0.29, NS) and .DELTA.AVP (r = 0.43, NS). The following hypotheses are supported. With exercise, AVP release is a primary factor for fluid and electrolyte regulation as it is highly correlated with the plasma hyperosmolality produced by a net hypotonic plasma efflux. An exercise intensity greater than 40% .ovrhdot.VO2 max is required to change plasma osmolality and, thus, stimulate significant AVP release. The stimulation of the renin-angiotensin system is a more general stress response, which responds to increasing sympathetic nervous activity.